This invention relates to integrated circuit input buffer circuits. More specifically, a circuit constructed in accordance with this invention allows an integrated circuit to accept a large negative voltage as an input signal. One example of a specific application of this large input signal buffering circuit is as an input buffer for a piezoelectric transducer input signal.
A piezoelectric transducer is a device which converts a physical force, i.e. a "push" or a "pull", directly into an electrical signal. The operation of a piezoelectric transducer can be likened to a charged capacitor which has movable plates. When the plates of this fictitious capacitor are pulled apart, the capacitance decreases. The voltage across a capacitor is determined by the equation,
V=Q/C where, PA1 V is the voltage across the capacitor, PA1 Q is the electric charge stored in the capacitor, and PA1 C is the capacitance of the capacitor.
For a given electrical charge, when the value of C decreases, the voltage across the capacitor increases. Depending on the force applied to the piezoelectric transducer, the voltage across the piezoelectric transducer may be very large, possibly as much as 200 volts. Typical bipolar devices, such as a 2N3906 PNP device, have breakdown voltages of approximately 40 volts. Typical metal oxide semiconductor (MOS) devices have breakdown voltages of approximately 30 volts. Therefore, the voltage provided by a piezoelectric transducer cannot be directly placed between any of the leads of a bipolar or MOS device without destroying that device.
Previous methods for receiving high voltage input signals have used nonintegrated heavy duty resistors as a voltage divider network to reduce the level of the voltage transmitted to the integrated circuit. See Mellen, et al., "Low Noise-High Gain JFET Amplifier For A Piezoelectric Transducer", U.S. Pat. No. 4,214,215, which is hereby incorporated by reference. The use of nonintegrated components is undesirable in that each nonintegrated component must by separately installed in the circuit and separately constructed, thereby increasing the cost of manufacturing the circuit. Accordingly, one goal of the present invention is to provide means for buffering a high voltage input signal using a minimum of components external to an integrated circuit.